Composite positive electrode material and method for making same

ABSTRACT

A composite positive electrode material for use in electrochemical cells. The composite material includes a particle of positive electrode material and a nucleating particle at least partially embedded within the interior of the particle of positive electrode material.

RELATED APPLICATION INFORMATON

[0001] This application is a divisional application of U.S. patentapplication Ser. No. 09/135,460 filed on Aug. 17, 1998.

FIELD OF THE INVENTION

[0002] The instant invention relates generally to positive electrodematerials for rechargeable batteries such as nickel hydroxide materials.More specifically, the instant invention relates to composite nickelhydroxide particulate having increased conductivity over the prior artmaterial.

BACKGROUND OF THE INVENTION

[0003] In rechargeable alkaline cells, weight and portability areimportant considerations. It is also advantageous for rechargeablealkaline cells to have long operating lives without the necessity ofperiodic maintenance. Rechargeable alkaline cells are used in numerousconsumer devices such as calculators, portable radios, and cellularphones. They are often configured into a sealed power pack that isdesigned as an integral part of a specific device. Rechargeable alkalinecells can also be configured as larger cells that can be used, forexample, in industrial, aerospace, and electric vehicle applications.

[0004] There are many known types of Ni based cells such as nickelcadmium (“NiCd”), nickel metal hydride (“Ni—MH”), nickel hydrogen,nickel zinc, and nickel iron cells. NiCd rechargeable alkaline cells arethe most widely used although it appears that they will be replaced byNi—MH cells. Compared to NiCd cells, Ni—MH cells made of syntheticallyengineered materials have superior performance parameters and contain notoxic elements.

[0005] Ni—MH cells utilize a negative electrode that is capable of thereversible electrochemical storage of hydrogen. Ni—MH cells usuallyemploy a positive electrode of nickel hydroxide material. The negativeand positive electrodes are spaced apart in the alkaline electrolyte.Upon application of an electrical potential across a Ni—MH cell, theNi—MH material of the negative electrode is charged by theelectrochemical absorption of hydrogen and the electrochemical dischargeof a hydroxyl ion, as shown in equation (1):

[0006] The negative electrode reactions are reversible. Upon discharge,the stored hydrogen is released to form a water molecule and release anelectron. The reactions that take place at the nickel hydroxide positiveelectrode of a Ni—MH cell are shown in equation (2):

[0007] Ni—MH materials are discussed in detail in U.S. Pat. No.5,277,999 to Ovshinsky, et al., the contents of which are incorporatedby reference.

[0008] In alkaline rechargeable cells, the discharge capacity of anickel based positive electrode is limited by the amount of activematerial, and the charging efficiencies. The charge capacities of a Cdnegative electrode and a MH negative electrode are both provided inexcess, to maintain the optimum capacity and provide overchargeprotection. Thus, a goal in making the nickel positive electrode is toobtain as high an energy density as possible. The volume of a nickelhydroxide positive electrode is sometimes more important than weight.The volumetric capacity density is usually measured in mAh/cc andspecific capacity is written as mAh/g.

[0009] At present, sintered or pasted nickel hydroxide positiveelectrodes are used in NiCd and Ni—MH cells. The process of makingsintered electrodes is well known in the art. Conventional sinteredelectrodes normally have an energy density of around 480-500 mAh/cc. Inorder to achieve significantly higher capacity, the current trend hasbeen away from sintered positive electrodes and toward foamed and pastedelectrodes.

[0010] Sintered nickel electrodes have been the dominant nickelelectrode technology for several decades for most applications. Theseconsist of a porous nickel plaque of sintered high surface area nickelparticles impregnated with nickel hydroxide active material either bychemical or electrochemical methods. While expensive, sinteredelectrodes provide high power, high reliability, and high cycle life,but not the highest energy density. They are likely to remain importantfor high reliability military and aerospace applications for some time.

[0011] Pasted nickel electrodes consist of nickel hydroxide particles incontact with a conductive network or substrate, preferably having a highsurface area. There have been several variants of these electrodesincluding the so-called plastic-bonded nickel electrodes which utilizegraphite as a microconductor and also including the so-called foam-metalelectrodes which utilize high porosity nickel foam as a substrate loadedwith spherical nickel hydroxide particles and cobalt conductivityenhancing additives. Pasted electrodes of the foam-metal type nowdominate the consumer market due to their low cost, simplemanufacturing, and higher energy density relative to sintered nickelelectrodes.

[0012] Conventionally, the nickel battery electrode reaction has beenconsidered to be a one electron process involving oxidation of divalentnickel hydroxide to trivalent nickel oxyhydroxide on charge andsubsequent discharge of trivalent nickel oxyhydroxide to divalent nickelhydroxide, as shown in equation 2 hereinbelow.

[0013] Some recent evidence suggests that quadrivalent nickel isinvolved in the nickel hydroxide redox reaction. This is not a newconcept. In fact, the existence of quadrivalent nickel was firstproposed by Thomas Edison in some of his early battery patents. However,full utilization of quadrivalent nickel has never been investigated.

[0014] In practice, electrode capacity beyond the one-electron transfertheoretical capacity is not usually observed. One reason for this isincomplete utilization of the active material due to isolation ofoxidized material. Because reduced nickel hydroxide material has a highresistance, the reduction of nickel hydroxide adjacent the currentcollector forms a less conductive surface that interferes with thesubsequent reduction of oxidized active material that is farther away.

[0015] As discussed in U.S. Pat. No. 5,348,822, nickel hydroxidepositive electrode material in its most basic form has a maximumtheoretical specific capacity of 289 mAh/g, when one charge/dischargecycles from a βII phase to a βIII phase and results in one electrontransferred per nickel atom. It was recognized in the prior art thatgreater than one electron transfer could be realized by deviating fromthe βII and βIII limitations and cycling between a highly oxidizedγ-phase nickel hydroxide phase and the βII phase. However, it was alsowidely recognized that such gamma phase nickel hydroxide formationdestroyed reversible structural stability and therefore cycle life wasunacceptably degraded. A large number of patents and technicalliterature disclosed modifications to nickel hydroxide material designedto inhibit and/or prevent the destructive formation of the transition tothe γ-phase, even though the higher attainable capacity through the useof γ-phase is lost.

[0016] Attempts to improve nickel hydroxide positive electrode materialsbegan with the addition of modifiers to compensate for what wasperceived as the inherent problems of the material. The use ofcompositions such as NiCoCd, NiCoZn, NiCoMg, and their analogues aredescribed, for example, in the following patents:

[0017] U.S. Pat. No. Re. 34,752, to Oshitani, et al., reissued Oct. 4,1994, describes a nickel hydroxide active material that contains nickelhydroxide containing 1-10 wt % zinc or 1-3 wt % magnesium to suppressthe production of gamma-NiOOH. The invention is directed towardincreasing utilization and discharge capacity of the positive electrode.Percent utilization and percent discharge capacity are discussed in thepresence of various additives.

[0018] Oshitani, et al. describe the lengths that routineers in the artthought it was necessary to go to in order to inhibit γ-NiOOH. Thepatent states:

[0019] Further, since the current density increased in accordance withthe reduction of the specific surface area, a large amount of higheroxide γ-NiOOH may be produced, which may cause fatal phenomena such asstepped discharge characteristics and/or swelling. The swelling due tothe production of γ-NiOOH in the nickel electrode is caused by the largechange of the density from high density β-NIOOH to low density γ-NiOOH.The inventors have already found that the production of γ-NiOOH caneffectively be prevented by addition of a small amount of cadmium in asolid solution into the nickel hydroxide. However, it is desired toachieve the substantially same or more excellent effect by utilizingadditive other than the cadmium from the viewpoint of the environmentalpollution.”

[0020] U.S. Pat. No. 5,366,831, to Watada, et al., issued Nov. 22, 1994,describes the addition of a single Group II element (such as Zn, Ba, andCo) in a solid solution with nickel hydroxide active material. The GroupII element is described as preventing the formation of gamma phasenickel hydroxide thereby reducing swelling, and the cobalt is describedas reducing the oxygen overvoltage thereby increasing high temperaturecharging efficiency. Both oxygen overvoltage and charge efficiency aredescribed as increasing with increasing cobalt.

[0021] U.S. Pat. No. 5,451,475, to Ohta, et al., issued Sep. 19, 1995,describes the positive nickel hydroxide electrode material as fabricatedwith at least one of the following elements added to the surface of theparticles thereof: cobalt, cobalt hydroxide, cobalt oxide, carbonpowder, and at least one powdery compound of Ca, Sr, Ba, Cu, Ag, and Y.The cobalt, cobalt compound, and carbon are described as constituents ofa conductive network to improve charging efficiency and conductivity.The powdery compound is described as adsorbed to the surface of thenickel hydroxide active material where it increases the overvoltage, forevolution of oxygen, thereby increasing nickel hydroxide utilization athigh temperature. Ohta, et al. claims that increased utilization in NiMHcells using the disclosed invention remains constant up to a high numberof charge/discharge cycles and utilization does not drop as much athigher temperatures as it does in cells that do not embody theinvention.

[0022] U.S. Pat. No. 5,455,125 to Matsumoto, et al., issued Oct. 3,1995, describes a battery having a positive electrode comprising nickelhydroxide pasted on a nickel foam substrate with solid solution regionsof Co and salts of Cd, Zn, Ca, Ag, Mn, Sr, V, Ba, Sb, Y, and rare earthelements. The addition of the solid solution regions is intended tocontrol the oxygen overvoltage during charging. The further externaladdition of “electric conducting agents” such as powdered cobalt, cobaltoxide, nickel, graphite, “and the like,” is also described. Energydensity is shown as constant at 72 Wh/kg at 20° C. and 56 Wh/kg at 45°C. for embodiments of the invention over the life of the NiMH cell.

[0023] U.S. Pat. No. 5,466,543, to Ikoma, et al., issued Nov. 14, 1995,describes batteries having improved nickel hydroxide utilization over awide temperature range and increased oxygen overvoltage resulting fromthe incorporation of at least one compound of yttrium, indium, antimony,barium, or beryllium, and at least one compound of cobalt or calciuminto the positive electrode. Cobalt hydroxide, calcium oxide, calciumhydroxide, calcium fluoride, calcium peroxide, and calcium silicate arespecifically described compounds. Additionally described additives arecobalt, powdery carbon, and nickel. The specification particularlydescribes AA cells using a positive electrode containing 3 wt % zincoxide and 3 wt % calcium hydroxide as superior in terms of cycle life(250 cycles at 0° C., 370 cycles at 20° C., and 360 cycles at 40° C.)and discharge capacity (950 mAh at 20° C., 850 mAh at 40° C., and 780mAh at 50° C.).

[0024] U.S. Pat. No. 5,489,314, to Bodauchi, et al., issued Feb. 6,1996, describes mixing the nickel hydroxide positive electrode materialwith a cobalt powder compound followed by an oxidation step to form abeta cobalt oxyhydroxide on the surface of the nickel hydroxide powder.

[0025] U.S. Pat. No. 5,506,070, to Mori, et al., issued Apr. 9, 1996,describes nickel hydroxide positive electrode material containing 2-8 wt% zinc mixed with 5-15% cobalt monoxide. The zinc reduces swelling andthe cobalt increases utilization. The capacity of the resultingelectrode is stated as being “improved up to 600 mAh/cc” without furtherdescription.

[0026] U.S. 5,571,636, to Ohta, et al., issued Nov. 5, 1996, describesthe addition of at least one powdery compound of Ca, Sr, Ba, Cu, Ag, andY to the surface of nickel hydroxide active positive electrode material.This patent states that these compounds are adsorbed to the surface ofthe nickel hydroxide active material creating a conductive network thatincreases the oxygen overvoltage and improves utilization of the activematerial at high temperatures. Increased utilization in NiMH cells usingthe '636 invention remains constant up to a large number of cycles andutilization does not drop as much at higher temperatures as it does incells that do not embody the invention.

[0027] In all of the prior art, the basic nickel hydroxide material istreated, most commonly, by the addition of a single element, usually Cocompounds, to increase electrical conductivity and usually one otherelement, usually Cd or Zn, to suppress and/or prevent γ-phase formation.The mechanisms for the asserted improvements in all the above patentsare attributable to the following effects:

[0028] 1. Improved speed of activation, resistance to poisons, andmarginal capacity improvement via increased utilization. At the presenttime, most commercial nickel metal hydride batteries achieve theseeffects through the external addition of up to 5 wt % cobalt and/orcobalt-containing compound. It is generally believed that the majorreason cobalt is effective at these levels is because it creates anextensive external conductive network independent of the nickelhydroxide material. Frequently, powdered carbon, powdered cobalt metal,and powdered nickel metal are also added to create separate conductivenetworks and thereby improve utilization. Of course, a major drawback ofincreasing the amount of such additives is that the amount of activenickel hydroxide electrode material is correspondingly reduced, therebyreducing capacity. Further, since Co is expensive, the addition of evenminimum amounts of Co greatly increases cost.

[0029] 2. Cycle life is extended by decreasing swelling that isinitiated by density changes between the oxidized and reduced states ofthe nickel hydroxide material. Swelling, in turn, is accelerated by theuncontrolled density changes between βII-βIII phase nickel hydroxide andα-γ or βII-γ phase nickel hydroxide. Cd and Zn incorporated into thenickel hydroxide effectively reduce the swelling by reducing thedifference in density in the charged and discharged material andincreasing the mechanical stability of the nickel hydroxide materialitself. This is accomplished by promoting oxygen evolution and therebyreducing charge acceptance which prevents the nickel hydroxide materialfrom attaining the highly oxidized state (the γ-phase state). However,by suppressing or at least significantly inhibiting γ-phase formation,the nickel hydroxide is limited to transferring no more than oneelectron per Ni atom. Further, in order to effectively inhibit γ-phasenickel hydroxide, it is necessary to employ a relatively high wt % ofthe inhibitor element such as Zn or Cd, which high percentage results ina greatly reduced amount of active material being present therebyresulting in reduced electrochemical capacity.

[0030] 3. The aforementioned “safety release” mechanism of oxygenevolution to avoid highly oxidized states of nickel hydroxide materialactually is an impediment to high temperature operation because asignificant increase in the rate of oxygen evolution occurs withincreasing temperature. The effect of such increased oxygen evolution isa very substantial decrease in utilization and ultimately a reduction inenergy storage at higher temperatures in the NiMH battery using thesematerials. At 55° C., for example, run times of a battery may be reducedby 35-55% compared to the room temperature performance of that battery.

[0031] Elevated operational temperature conditions aside, none of theseprior art modifications result in more than an incremental improvementin performance and none result in a significant increase in the capacityof the nickel hydroxide material itself, even at room temperature.Further, these modifications fail to address the special operationalrequirements of NiMH batteries, particularly when NiMH batteries areused in electric vehicles, hybrid vehicles, scooters and other highcapacity, high drain rate applications. Because NiMH negative electrodeshave been improved and now exhibit an extremely high storage capacity,the nickel hydroxide positive electrode material is essentially thelimiting factor in overall battery capacity. This makes improving theelectrochemical performance of the nickel hydroxide material in allareas more important than in the past. Unfortunately, the elementscurrently added to the nickel hydroxide material result in insufficientimprovements in performance before competing deleterious mechanisms andeffects occur. For example, Cd cannot be used in any commercial batterybecause of the environmental impact thereof, and Co and Zn appear tobecome most effective only at levels that result in a significantdecrease in cell capacity; more specifically, energy per electrodeweight.

[0032] Ovshinsky and his team have developed positive electrodematerials that have demonstrated reliable transfer of more than oneelectron per nickel atom. Such materials are described in U.S. Pat. Nos.5,344,728 and 5,348,822 (which describe stabilized disordered positiveelectrode materials) and copending U.S. patent application Ser. No.08/300,610 filed Aug. 23, 1994, and U.S. patent application Ser. No.08/308,764 filed Sep. 19, 1994.

[0033] Previously all of the work on nickel hydroxide positive electrodematerial has concentrated on improving it's conductivity in two ways.First electrically conductive additives have been externally mixed withthe nickel hydroxide materials used to produce pasted electrodes. Suchadditives include Co, CoO, Ni, Cu, and C. The additives are generally inthe form of powder, fibers or the like. These techniques have achievedmoderate success in that Ni—MH batteries have achieved impressive gainsin high rate discharge performance. However, there are two remainingproblems. First, the emergence of hybrid electric vehicles has demandedthat Ni—MH batteries achieve 1000 W/kg of power. Conventional electricvehicle batteries achieve 250 W/kg and special designs achieve 500-600W/kg. Second, even present power levels are achieved by a very expensiveand elaborate positive electrode embodiment (i.e., an expensive foammetal skeleton and expensive use of cobalt compounds).

[0034] The second way in which artisans have increased the conductivityof nickel hydroxide is by co-precipitating cobalt hydroxide along withnickel hydroxide to increase it's internal conductivity. While NiCoco-precipitates have better conductivity and utilization than purenickel hydroxide, the improvement can only be considered incrementalwith no room for further improvement.

[0035] The two methods discussed above, while increasing the power andcapacity of the nickel hydroxide materials and electrodes have still notrealized the full potential thereof. As stated above, there is still aneed for significant gains in power and high rate discharge capability.Therefore, there is a need in the art for additional improvements in theconductivity of positive electrode materials and, specifically, in theconductivity of nickel hydroxide for use in rechargeable batteryapplications.

SUMMARY OF THE INVENTION

[0036] An objective of the present invention is to provide an activematerial for a positive electrode having increased conductivity. Thisand other objectives are satisfied by a composite positive electrodematerial for use in electrochemical cells. The material comprises aparticle of positive electrode material; and a conductive material atleast partially embedded within the particle of positive electrodematerial. The conductive material may be metallic particles such asnickel particles.

[0037] This and other objectives are satisfied by a method for producinga composite positive electrode material comprising a particle ofpositive electrode material and a conductive material at least partiallyembedded within the particle of positive electrode material, the methodcomprising the step of: combining a metal ion solution, a causticsolution, and a conductive material, whereby a precipitation solutionincluding the composite positive electrode material is formed. Thecombining step may comprise the steps of mixing the conductive materialwith the metal ion solution to form a suspension; and mixing thesuspension with the caustic solution.

[0038] This and other objectives are also satisfied by a compositepositive electrode material comprising: a particle of positive electrodematerial; and a nucleating particle at least partially embedded withinthe particle of positive electrode material.

[0039] This and other objectives are also satisfied by a method forproducing a composite positive electrode material comprising particlesof positive electrode material having nucleating particles at leastpartially embedded therein, the method comprising the step of: combininga metal ion solution, a caustic solution, and the nucleating particles,whereby a precipitation solution including the composite positiveelectrode material is formed. The combining step may comprise the stepsof: mixing the metal ion solution and the nucleating particles, therebyforming a suspension of the nucleating particles in the metal ionsolution; and mixing the caustic solution with the suspension.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1 shows a photomicrograph, at magnification of 10,000×, of anembodiment of the composite material of the present invention;

[0041]FIG. 2 is a stylized drawing of an embodiment of the compositematerial where the conductive material is in the form of a conductivenetwork;

[0042]FIG. 3 shows complex impedance plots for positive electrodescomprising nickel hydroxide material formed with and without embeddednickel particles; and

[0043]FIG. 4 shows discharge curves for positive electrodes comprisingnickel hydroxide material formed with and without embedded nickelparticles.

DETAILED DESCRIPTION OF THE INVENTION

[0044] The instant inventors have discovered improvements in positiveelectrode material for use in electrochemical cells and methods formaking the improved materials. Disclosed herein is a composite positiveelectrode material for use in electrochemical cells. Generally, thecomposite positive electrode material comprises a particle of positiveelectrode material, and a conductive material which is at leastpartially embedded within the particle of positive electrode material.The conductive material may be totally embedded within the particle ofpositive electrode material.

[0045] Generally, the conductive material is any material which iselectrically conductive. Preferably, the conductive material is chosenso that the conductivity of the composite positive electrode material isgreater than the conductivity of the active positive electrode materialalone.

[0046] The conductive material may comprise a metal. Examples of metalswhich may be used include, but are not limited to, nickel, nickelalloys, copper, and copper alloys. Preferably, the metal is nickel. Asused herein, “nickel” refers to substantially pure nickel. Also, as usedherein, “copper” refers to substantially pure copper.

[0047] It is noted that nickel has an atomic configuration comprisingd-orbitals. While not wishing to be bound by theory, it is believed thatthe d-orbitals may effect the active positive electrode materialsurrounding the nickel material.

[0048] The conductive material may also comprise a material selectedfrom the group consisting of oxides, nitrides, carbides, suicides, andborides. The conductive material may comprise carbon, or graphite. Theconductive material may comprise copper oxide, cobalt oxide, or indiumtin oxide.

[0049] The conductive material may be in the form of at least oneconductive particle which is at least partially embedded in the particleof positive electrode material. Preferably, the conductive particle ismetallic. More preferably, the conductive particle is a nickel particle.FIG. 1 shows a photomicrograph, at magnification of 10,000×, of anembodiment of the composite material of the present invention. In thisembodiment, the composite material comprises a particle of positiveelectrode material 1, and a nickel particle 3 which is totally embeddedin the particle of positive electrode material 1.

[0050] The conductive material may comprise a plurality of conductiveparticles which are at least partially embedded within the particle ofpositive electrode material. The plurality of conductive particles maybe isolated from one another. Alternately, at least some of theparticles may be touching others so as to form a conductive network ofparticles.

[0051] The conductive particles may have a variety of shapes and sizes.For example, the particles may be substantially spherical. Alternately,the particles may be elongated wherein one dimension is longer thananother dimension. For example, the particles may be ellipsoidal orcylindrical. As well, the particles may be in the form of threadlikefibers. These elongated particles may have an average length which isless than or equal to about 10 microns. As well they may have an averagediameter of less than or equal to about 1.0 micron. These sizes aremerely reference points and may be varied within the scope of theinvention. An example of conductive particles are the INCO T-210 nickelparticles. The INCO T-210 nickel particles have a particle morphologywith an average sub-micron Fisher diameter of about 0.9 microns, anapparent density of about 0.6 grams per cm³, and a BET of about 1.75m²/g.

[0052] The conductive material may take the form of a conductivenetwork. The conductive network may have various topologies. One exampleof a conductive network is a lattice structure which may be formed bythe interconnection of conductive particles, fibers, strands, and thelike. Another example of a conductive network is the branching tree-likestructure that is shown in FIG. 2. The conductive network 3A branchesout throughout the active positive electrode particle 1. Another exampleof a conductive network is one comprising one or more carbon nanotubesand/or fullerenes.

[0053] While not wishing to be bound by theory, the inventors believethat the embedded conductive material, such as the nickel particle 3shown in FIG. 1, serves one or both of two possible roles in thecomposite material. First, the inventors believe that the conductivematerial serves as an electronically conductive pathway through theactive positive electrode material, thereby increasing the useablecapacity of the active material. The internal conductive pathway alsoimproves the ionic transport within the active material and preventsportions of the active material from becoming electrically isolated byreducing the transport distance through the active material and/oroptimizing alignment of crystallite pathways.

[0054] The present inventors believe that one factor that limits thenickel electrode reaction to capacities equivalent to one electron orless is the underutilization of the nickel hydroxide active material. Itis believed that the underutilization is caused by electronic isolationof oxidized nickel oxyhydroxide (NiOOH) material by the formation ofhighly resistive nickel hydroxide (Ni(OH)₂) material adjacent to theactive material, and by poor transport of ions to the inner portions ofthe electrode which are remote from the electrolyte. The presentinvention, overcomes such electronic isolation and ionic transportlimitations. Hence, in this invention, electronic isolation of theactive material is reduced or avoided by providing an electronicallyconductive pathway in the interior of the nickel hydroxide particles.This allows for added electronic pathways which reduce or preventisolation of the active material by the more resistive reduced nickelhydroxide material.

[0055] The second role that the conductive material, such as the nickelparticle 3, may play is that of a nucleation site for the growth ofnickel hydroxide crystallites. The particles of nickel hydroxidematerial comprise crystallites, and the nickel particle 3 behaves as a“nucleating particle” (i.e., a nucleation site for the growth of thenickel hydroxide crystallites). As a nucleation site, the nickelparticle 3 may orient the nickel hydroxide crystallites as they depositonto the nickel particle during precipitation. Furthermore, the nickelparticle 3 may also influence the size and/or shape of the nickelhydroxide crystallites. Each nickel hydroxide particle is composed ofmany very fine crystallites which may have an improved crystallographicorientation within the boundary of the crystallite.

[0056] The protonic conductivity (i.e., the conductivity of protons) ina typical nickel hydroxide particle is dominated by (1) conductionwithin crystallites and (2) conduction across the grain boundariesbetween adjacent crystallites. When the crystallite size is too large,the fully discharged nickel hydroxide does not have enough vacancies,created at the grain boundaries for the initial charging current toprovide for a proton to hop from one vacancy to another vacancy, andtherefore such large crystallites provide for relatively poorconductivity. When the crystallite size is too small, the adjacentcrystal lattice conduction networks will not be aligned due to thepresence of too many grain boundary vacancies for the protons to hopacross and protonic conductivity is thereby impeded. There exists anoptimum crystallite size in which the proper number of grain boundaryvacancies are present. In these latter materials, sufficient vacanciesare present for proper inter-crystallite conductivity to occur and theprotons have sufficient room to follow the proper conductive paththrough the crystallite it then enters.

[0057] In addition to the proper crystallite size of the nickelhydroxide material, the crystallites are believed to require properorientation to be highly conductive. That is, if there arediscontinuities in the crystallite orientation from one crystallite toanother then the crystallites that are improperly oriented for lowerresistance current flow will dominate the resistance of the material.Conversely, if all of the crystallites are properly oriented, theconductivity of the nickel hydroxide material may be increased. Theinventors believe that the nickel particle 3 may preferentially orientthe crystallites of nickel hydroxide in this highly conductiveorientation as they deposit, such that the nickel hydroxide has a higherprotonic conductivity than nickel hydroxide deposited in a randommanner.

[0058] Thus, it is possible that the addition of nucleation sites alterthe size and/or shape of the crystallites. In a random precipitation, itmight be expected that crystallites would have a spherical shape, whilein the present invention the crystallites could have a more elongatedshape. While not wishing to be bound by theory, it is possible thatprotonic conduction is preferential along the 101 axis of the nickelhydroxide. The role of the nucleation sites could be to reduce thedistance along the 101 plane to the crystallite boundary or to orientthe 101 planes from one crystallite to an other for enhance conduction.

[0059] Hence, an alternate embodiment of the present invention, is acomposite positive electrode material comprising a particle of activepositive electrode material (such as nickel hydroxide), and a“nucleating particle” which is at least partially embedded within theparticle of positive electrode material. The nucleating particle doesnot have to be an electrically conductive material. Instead, thenucleating particle need only provide a nucleation site for the growthof crystallites forming from the positive electrode material. Asdiscussed above, the addition of nucleation sites may orient thedeposited crystallites of the positive electrode material such that theconductivity of the material is increased. The nucleation sites mayprovide for the proper deposition surface to orient the crystallites aswell as to determine the average size and/or shape of the crystallitesso as to increase the conductivity of the material.

[0060] The nucleating particles are not limited to any specific shape,size or topology. Examples of shapes include, but not limited to,substantially spherical, substantially flat, elongated, cylindrical,ellipsoidal, fiber-like, cubic, parallelopiped, etc. As well, thesurfaces of the nucleating particles may be varied to effect the growthof the positive electrode material crystals. For example, the nucleatingparticles may be partially etched to provide either a roughened surfaceor an oxide free surface.

[0061] Further, it may also be possible to provide a “nucleationstructure” having a more complex topology that a single nucleatingparticle and which also acts as a surface for the growth of crystals ofpositive electrode material. For example, the nucleation structure maybe a plurality of connected nucleating particles. The nucleationstructure may have the form of a lattice such as a matrix, screen orfoam. The nucleation structure may have a topology similar to that ofthe conductive network shown in FIG. 2. As well, the nucleationstructure may have a topology sufficient to increase the conductivity ofthe positive electrode material by appropriately orienting the growth ofthe crystals and/or appropriately determining the size of the crystalsand/or the shape of the crystals.

[0062] The active positive electrode material used in the presentinvention may be may be any type of positive electrode material known inthe art. Examples include nickel hydroxide material and manganesehydroxide material. It is within the spirit and intent of this inventionthat any and all kinds of nickel hydroxide, or positive materials ingeneral, may be used. Even pure nickel hydroxide without cobalt, amaterial with poor conductivity for commercial application, may betransformed into a viable positive electrode material via the internallyembedded nickel particles or fibers described herein.

[0063] The nickel hydroxide material may be a disordered material. Theuse of disordered materials allow for permanent alteration of theproperties of the material by engineering the local and intermediaterange order. The general principals are discussed in U.S. Pat. No.5,348,822, the contents of which are incorporated by reference herein.The nickel hydroxide material may be compositionally disordered.“Compositionally disordered” as used herein is specifically defined tomean that this material contains at least one compositional modifierand/or a chemical modifier. Also, the nickel hydroxide material may alsobe structurally disordered. “Structurally disordered” as used herein isspecifically defined to mean that the material has a conductive surfaceand filamentous regions of higher conductivity, and further, that thematerial has multiple or mixed phases where alpha, beta, and gamma-phaseregions may exist individually or in combination.

[0064] The nickel hydroxide material may comprise a compositionally andstructurally disordered multiphase nickel hydroxide host matrix whichincludes at least one modifier chosen from the group consisting of Al,Ba, Bi, Ca, Co, Cr, Cu, F, Fe, In, K, La, Li, Mg, Mn, Na, Nd, Pb, Pr,Ru, Sb, Sc, Se, Sn, Sr, Te, Ti, Y, and Zn. Preferably, the nickelhydroxide material comprises a compositionally and structurallydisordered multiphase nickel hydroxide host matrix which includes atleast three modifiers chosen from the group consisting of Al, Ba, Bi,Ca, Co, Cr, Cu, F, Fe, In, K, La, Li, Mg, Mn, Na, Nd, Pb, Pr, Ru, Sb,Sc, Se, Sn, Sr, Te, Ti, Y, and Zn. These embodiments are discussed indetail in commonly assigned U.S. Pat. No. 5,637,423 the contents ofwhich is incorporated by reference herein.

[0065] The nickel hydroxide materials may be multiphase polycrystallinematerials having at least one gamma-phase that contain compositionalmodifiers or combinations of compositional and chemical modifiers thatpromote the multiphase structure and the presence of gamma-phasematerials. These compositional modifiers are chosen from the groupconsisting of Al, Bi, Co, Cr, Cu, Fe, In, LaH₃, Mg, Mn, Ru, Sb, Sn,TiH₂, TiO, Zn. Preferably, at least three compositional modifiers areused. The nickel hydroxide materials may include the non-substitutionalincorporation of at least one chemical modifier around the plates of thematerial. The phrase “non-substitutional incorporation around theplates”, as used herein means the incorporation into interlamellar sitesor at edges of plates. These chemical modifiers are preferably chosenfrom the group consisting of Al, Ba, Ca, Co, Cr, Cu, F, Fe, K, Li, Mg,Mn, Na, Sr, and Zn.

[0066] As a result of their disordered structure and improvedconductivity, the nickel hydroxide materials do not have distinctoxidation states such as 2⁺, 3⁺, or 4⁺. Rather, these materials formgraded systems that pass 1.0 to 1.7 and higher electrons.

[0067] The nickel hydroxide material may comprise a solid solutionnickel hydroxide material having a multiphase structure that comprisesat least one polycrystalline gamma-phase including a polycrystallinegamma-phase unit cell comprising spacedly disposed plates with at leastone chemical modifier incorporated around said plates, said plateshaving a range of stable intersheet distances corresponding to a 2⁺oxidation state and a 3.5⁺, or greater, oxidation state; and at leastthree compositional modifiers incorporated into the solid solutionnickel hydroxide material to promote the multiphase structure. Thisembodiment is fully described in commonly assigned U.S. Pat. No.5,348,822, the contents of which is incorporated by reference herein.

[0068] Preferably, one of the chemical modifiers is chosen from thegroup consisting of Al, Ba, Ca, Co, Cr, Cu, F, Fe, K, Li, Mg, Mn, Na,Sr, and Zn. The compositional modifiers may be chosen from the groupconsisting of a metal, a metallic oxide, a metallic oxide alloy, a metalhydride, and a metal hydride alloy. Preferably, the compositionalmodifiers are chosen from the group consisting of Al, Bi, Co, Cr, Cu,Fe, In, LaH₃, Mn, Ru, Sb, Sn, TiH₂, TiO, and Zn. In one embodiment, oneof the compositional modifiers is chosen from the group consisting ofAl, Bi, Co, Cr, Cu, Fe, In, LaH₃, Mn, Ru, Sb, Sn, TiH₂, TiO, and Zn. Inanother embodiment, one of the compositional modifiers is Co. In analternate embodiment, two of the compositional modifiers are Co and Zn.The nickel hydroxide material may contain 5 to 30 atomic percent, andpreferable 10 to 20 atomic percent, of the compositional or chemicalmodifiers described above.

[0069] The disordered nickel hydroxide electrode materials may includeat least one structure selected from the group consisting of (i)amorphous; (ii) microcrystalline; (iii) polycrystalline lacking longrange compositional order; and (iv) any combination of these amorphous,microcrystalline, or polycrystalline structures. A general concept ofthe present invention is that a disordered active material can moreeffectively accomplish the objectives of multi-electron transfer,stability on cycling, low swelling, and wide operating temperature thanprior art modifications.

[0070] Also, the nickel hydroxide material may be a structurallydisordered material comprising multiple or mixed phases where alpha,beta, and gamma-phase region may exist individually or in combinationand where the nickel hydroxide has a conductive surface and filamentousregions of higher conductivity.

[0071] Additional improvement of the nickel hydroxide material of thepresent invention are possible when these disordered materials arecombined with electrolytes where the electrolyte comprises at least oneelement chosen from the group consisting of Ba, Ca, Cs, K, Li, Na, Ra,Rb, and Sr, combined with at least one member of the group consisting ofBr, Cl, F, OH. Particular examples of such electrolytes are formulationsof KOH, NaOH, LiOH and/or CsF, and KOH and CsOH.

[0072] Also disclosed herein is a method for producing a compositepositive electrode material comprising a particle of positive electrodematerial, and a conductive material at least partially embedded withinthe particle of positive electrode material. The general method formaking the composite material is by precipitation of a positiveelectrode material (such as the nickel hydroxide material) onto theconductive material suspended in a precipitation bath. The specificmethod can be varied widely, as will be described hereinbelow, as longas the positive electrode material is deposited onto the conductivematerial.

[0073] The method requires a source of metal ion solution, a source ofthe conductive material, and a source of caustic (sodium hydroxide) beprovided. Generally, the method comprises the step of combining themetal ion solution, the caustic solution and the conductive material sothat a precipitation solution which includes the composite positiveelectrode material is formed.

[0074] A major proportion of the metal ion solution should include theactive materials main metal ion, for instance nickel ions, fordeposition of a nickel hydroxide material. While nickel ions aretypically used, manganese ions (for deposition of a manganese hydroxidesolution) may also be used. Also, other metal ions may be added to themetal ion solution to modify and enhance the performance of the nickelhydroxide material. The metal ion solution may further comprise one ormore metal ions selected from the group consisting of Al, Ba, Bi, Ca,Co, Cr, Cu, Fe, In, K, La, Li, Mg, Mn, Na, Nd, Pb, Pr, Ru, Sb, Sc, Se,Sn, Sr, Te, Ti, Y, and Zn. The metal ion solution may be selected fromthe group consisting of a metal sulfate solution, a metal nitratesolution, and mixtures thereof.

[0075] The caustic solution is generally a very concentrated sodiumhydroxide solution, and is standard in the art of nickel hydroxideprecipitation. As with prior art precipitation processes, the sodiumhydroxide can be partially replaced by hydroxides of other alkali metalhydroxides, as long as they are soluble.

[0076] In one embodiment, the method of producing the composite materialcomprises the step of mixing the conductive material with the metal ionsolution to form a suspension. The suspension is then mixed with thecaustic solution in a reactor vessel. Hence, in this embodiment, theconductive material is suspended in the metal ion solution before beingmixed with the caustic.

[0077] The conductive material is preferably nickel particles (which maybe fibers). It is noted that while the remaining discussion of themethod of making the composite material is in terms of nickel particles,all types of conductive materials (as discussed hereinabove) may beused.

[0078] Once suspended in the metal ion solution, the nickel particlesact as nucleation sites for the precipitation of the positive electrodeactive material (hereinafter nickel hydroxide material). After thesuspension is formed, the caustic solution is then mixed with thesuspension to precipitate the nickel hydroxide material onto the nickelparticle, thereby forming the deposit. As the nickel hydroxide depositsonto the nickel particle, the nickel particle becomes at least partiallyembedded in the nickel hydroxide material.

[0079] The inventors have noted the preferred aspect of adding thenickel particles to the reactor vessel by first suspending the nickelparticles in the metal ion solution, especially when the metel ionsolution is predominately a nickel sulfate solution. When added in thismanner, nucleation and precipitation proceeded excellently. In anearlier trial (in which the metal ion solution was also predominately anickel sulfate solution) the nickel particles were added independentlyto the reactor vessel. This case was unsuccessful, resulting in clumpedmetallic nickel particles outside of the nickel hydroxide. While notwishing to be bound by theory, the inventors believe that suspending thenickel particles in the metal ion solution may be preferred due to theacidic nature of the sulfate solution. It may be that the nickelparticles are partially etched, providing either a roughened surface oran oxide free surface better for nucleation. When ammonium hydroxide isadded to form a nickel ammonia complex (discussed below), it is alsopossible that the nickel ammonia complex which forms just prior to orsimultaneous with the precipitation assists or promotes the desirablenucleation. It is still possible that direct introduction of the nickelparticles to the precipitation reactor could work through the use of awetting agent or other means.

[0080] In another embodiment of the method, a source of ammoniumhydroxide is also provided. The ammonium hydroxide is mixed with themetal ion solution to form an amine complex with the metal ions. Theamine complex is then reacted with the caustic solution to form thenickel hydroxide material. The step of mixing the ammonium hydroxidesolution with the metal ion solution may occur before or concurrent withthe step of mixing the metal ion solution and the nickel particles. Thestep of mixing the ammonium hydroxide solution with the metal ionsolution may also occur after the step of mixing the metal ion solutionand the nickel particles, but before the step of mixing the causticsolution with the suspension. Finally, the step of mixing the ammoniumhydroxide solution with the metal ion solution may occur concurrent withthe step of mixing the caustic solution with the suspension.

[0081] The method of the present invention may further comprise the stepof separating the composite positive electrode material from theprecipitation solution. The composite positive electrode material may bewashed with deionized water and/or caustic solution.

[0082] The concentrations of the solutions are variable and generallyknown in the art. The nickel particles may form about 0.1% to about 35%by weight of the final nickel hydroxide powder. Present results indicatethat the effect of the added nickel particles can be seen to start atabout 2% by weight of nickel particles. After about 20% by weight, thereduction in active material is not compensated for by either theincrease in conductivity or the decrease in isolated active material.More preferably, the nickel particles form about 2% to about 10% byweight of the nickel hydroxide powder.

EXAMPLE

[0083] A composite nickel hydroxide material was prepared by mixing ametal ion solution, a calcium nitrate solution, a caustic solution ofNaOH, and an ammonium hydroxide solution in a reactor vessel.

[0084] The metal ion solution was prepared by adding 177 grams of CoSO₄,15.5 grams MgSO₄ and 11.2 grams of ZnSO₄ to 0.058 gallons of water and1.25 gallons of NiSO₄ solution. About 50 grams of the INCO T-210 nickelpowder was added to the metal ion solution as a source of metallicnickel particles and continuously stirred.

[0085] The metal ion solution is added to the reactor vessel at a rateof about 0.058 gallons per hour. A solution of 66% calcium nitrate isconcurrently added to the reactor vessel at a rate of about 0.0025gallons per hour. Ammonium hydroxide is added to the reaction vessel ata rate of about 0.016 gallons per hour. Finally, a caustic solutioncomprising about 0.96 gallons of a 6.5 M solution of NaOH is added toreaction vessel at a rate which is sufficient to keep the pH of thereaction vessel at about 11.3.

[0086] The reaction vessel is kept at a temperature of about 60° C. andis stirred at a rate of about 670 revolution per minute. The quantitiesof metal ion solution, calcium nitrate, caustic, and ammonium hydroxideused are sufficient to produce about 1 Kg of the composite nickelhydroxide material over a 24 hour period of time. After the compositenickel hydroxide material is formed, it is rinsed with deionized wateror dilute caustic, and dried.

[0087] Also disclosed herein is a method for producing a compositepositive electrode material comprising particles of active positiveelectrode material having nucleating particles at least partiallyembedded therein. The method comprises the step of combining a metal ionsolution, a caustic solution, and the nucleating particles, whereby aprecipitation solution including the composite positive electrodematerial is formed. In general, the method for producing the compositepositive electrode material with nucleating particles is the same as themethod discussed above with regards to using the conductive material.Nucleating particles (which need not be conductive) are used instead ofthe conductive material. Of course, the nucleating particles may beelectrically conductive particles such as nickel particles.

[0088]FIG. 3 shows the AC impedance measurements for positive electrodescomprising two different positive electrode materials. In general, theAC impedance measurements of a positive electrode is a plot showing thereal portion of electrode impedance on the horizontal axis and theimaginary portion of electrode impedance on the vertical axis. Theimpedances are plotted as a function of a range of frequencies startingat a high frequency of about 10 kHz and going to a low frequency ofabout 20 uHz.

[0089] Referring to FIG. 3, Plot A is the AC impedance measurements of apositive electrode comprising a nickel hydroxide active material. Plot Bis the AC impedance measurement of a positive electrode comprising thecomposite positive electrode material of the present invention. Thecomposite material comprises the same nickel hydroxide active material(from which Plot A was made) with the addition of about 5% by weight ofembedded nickel particles. An important electrical parameter of abattery electrode is the “charge transfer resistance”, R_(CT). Thecharge transfer resistance is calculated from the nyquist plotdescribing the AC impedance of the electrode over a range offrequencies.

[0090] The charge transfer resistance corresponds to the diameter of the“high frequency semicircle” of the AC impedance plot multiplied by thenumber of grams of positive electrode material. Referring to FIG. 3,this diameter is denoted as Diam(A) for Plot A and Diam(B) for Plot B.The Diam(A) was measured to be about 0.113 ohms while the number ofgrams of the positive electrode material (nickel hydroxide withoutembedded nickel) was about 2.85 grams. Hence, the charge transferresistance RCT(A) of the nickel hydroxide material without embeddednickel was about 0.322 ohms-gram.

[0091] The diameter of the semi-circle of Plot B, Diam(B) was measuredto be about 0.062 ohms while the number of grams of the positiveelectrode material (the same nickel hydroxide material with 5% embeddedNi) was about 2.94 grams. Hence, the charge transfer resistanceR_(CT)(B) of the nickel hydroxide material with the 5% embedded nickelfibers was about 0.182 ohms-gram. Hence, the charge transfer resistancefor the nickel hydroxide material with embedded nickel, R_(CT)(B), wassignificantly lower than the charge transfer resistance for the nickelhydroxide material without the embedded nickel R_(CT)(A). The additionof the nickel particles to the nickel hydroxide material may lower thecharge transfer resistance of the positive electrode material by over50%.

[0092] Also disclosed herein is a positive electrode materialcharacterized by a charge transfer resistance less than about 0.22ohms-gram. Preferably, the positive electrode material has a chargetransfer resistance is less than about 0.20 ohms-gram. Most preferably,the charge transfer resistance is less than about 0.19 ohms-gram.

[0093]FIG. 4 shows positive electrode discharge curves A and B. Thedischarge curves A, B show the positive electrode half cell potentialsrelative to an Hg/HgO reference electrode. The potentials are given fromabout 95% state of charge to about 50% state of charge. Discharge curveA is for a positive electrode having an active electrode material whichcomprises a nickel hydroxide material without any embedded nickelparticles. Discharge curve B is for a positive electrode having anactive material comprising the same nickel hydroxide material with about5% by weight of embedded nickel fibers (i.e., about 5% of the INCO T-210nickel particles). Comparison of the discharge curves shows that theaddition of the nickel fibers increases the half cell potential of thepositive electrode over the entire range of state of discharge (i.e.,from 95% to 50%). Though not wishing to be bound by theory, it isbelieved that the increased potential is at least partially due to thedecreased charge transfer resistance discussed above.

[0094] Capacity and utilization are also greatly enhanced by the presentinvention. Positive electrodes were prepared for half-cell testing bypasting a slurry of about 5% by weight of Co metal, about 5% by weightof CoO with PVA binder and the remainder active material onto foam metalsubstrates.

[0095] The respective electrode samples included approximately 2 gramsof active material paste (including binder and external additives, butnot including the foam metal substrate) and were tested in an excesselectrolyte configuration with the following results. “Sample A” is anickel hydroxide positive electrode material without any embedded nickelfibers. “Sample B” is the same nickel hydroxide material with about 5%by weight of the embedded nickel fibers (5% of INCO T-210 nickelparticles). “Commercial” is a commercially available material. Thecapacities of the positive electrode materials are shown in the Tablebelow. TABLE Sample mAh/g Commercial 230 A (without embedded nickel) 276B (with 5% embedded nickel) 292

[0096] As can be seen from the Table, the inclusion of the 5% nickelfibers in the nickel hydroxide material greatly enhances the utilizationand capacity of the active material.

[0097] It is noted that the composite positive electrode material of thepresent invention may provide for a “fully pasted positive electrode”that does not use a foam or fiber skeleton or substrate. Usually, thisembodiment is formed from a standard nickel hydroxide material withexternally conductive additives and a plastic binder. The compositeelectrode material of the present invention could provide the improvedpower and rate discharge needed for practical commercialization of thistype of electrode.

[0098] The term “substrate” as used herein relates to any electricallyconductive support for the active positive electrode material. It maytake the form of a foam, grid, plate, foil, expanded metal or any othertype of support structure. It may take the form of conventional nickelfoils, plates and foams, as well as, carbon networks, fibers orparticulate and cobalt oxyhydroxide networks. It may be made from anyelectronically conductive material. Preferably, it is made from a metalsuch as nickel or a nickel alloy. More preferably, the substrate for thepositive electrode is a nickel foam.

[0099] It is to be understood that the disclosure set forth herein ispresented in the form of detailed embodiments described for the purposeof making a full and complete disclosure of the present invention, andthat such details are not to be interpreted as limiting the true scopeof this invention as set forth and defined in the appended claims.

We claim:
 1. A composite positive electrode material comprising: aparticle of positive electrode material; and a nucleating particle atleast partially embedded within the interior of said particle ofpositive electrode material.
 2. The composite positive electrodematerial of claim 1 , wherein said nucleating particle orients thecrystallites of said particle of positive electrode material so that theconductivity of said material is increased.
 3. The composite positiveelectrode material of claim 1 , wherein said nucleating particledetermines the average size of the crystallites of said particle ofpositive electrode material so that the conductivity of said material isincreased.
 4. The composite positive electrode material of claim 1 ,wherein said nucleating particle determines the average shape of thecrystallites of said particle of positive electrode material so that theconductivity of said material is increased.
 5. The composite positiveelectrode material of claim 1 , wherein said nucleating particle istotally embedded within said particle of positive electrode material. 6.The composite positive electrode material of claim 1 , wherein saidnucleating particle comprises a conductive material.
 7. The compositepositive electrode material of claim 6 , wherein said conductivematerial comprises a metal.
 8. The composite positive electrode materialof claim 6 , wherein said conductive material comprises nickel.
 9. Thecomposite positive electrode material of claim 6 , wherein saidconductive material comprises a nickel alloy.
 10. The composite positiveelectrode material of claim 6 , wherein said conductive materialcomprises copper.
 11. The composite positive electrode material of claim6 , wherein said conductive material comprises a copper alloy.
 12. Thecomposite positive electrode material of claim 6 , wherein saidconductive material comprises at least one material selected from thegroup consisting of oxides, nitrides, carbides, suicides, and borides.13. The composite positive electrode material of claim 6 , wherein saidconductive material comprises at least one material selected from thegroup consisting of carbon, and graphite.
 14. The composite positiveelectrode material of claim 6 , wherein said conductive materialcomprises at least one material selected from the group consisting ofcopper oxide, cobalt oxide, and indium tin oxide.
 15. The compositepositive electrode material of claim 1 , wherein said nucleatingparticles is elongated.
 16. The composite positive electrode material ofclaim 1 , wherein said nucleating particle is substantially spherical.17. The composite positive electrode material of claim 1 , wherein saidnucleating particle is a fiber.
 18. The composite positive electrodematerial of claim 1 , wherein said nucleating particle has an averagelength of less than about 10 microns.
 19. The composite positiveelectrode material of claim 1 , wherein said nucleating particle has anaverage diameter of less than about 1.0 micron.
 20. The compositepositive electrode material of claim 1 , wherein said positive electrodematerial comprises nickel hydroxide material.